1. Field of the Invention
The present invention relates to an apparatus for removing nitrogen and phosphorus in a waste water treatment plant, and more particularly, to an apparatus for removing nitrogen and phosphorus by combining at least two sets of unit systems constructed by incorporating a clarifier into an oxidation ditch and operating the unit systems by a method of intermittent aeration and change of flow path.
2. Description of the Related Art
To remove nitrogen and phosphorus biologically in a waste water treatment plant, an anoxic process in which free oxygen is not supplied, an anaerobic process, and an aerobic process in which oxygen is supplied, are carried out. In an aerobic reaction basin, organic nitrogen and ammoniacal nitrogen are oxidized into nitrate. In an anoxic reaction basin, a denitrification process in which the nitrate is reduced into nitrogen gas to then be released to the air is carried out. In an anaerobic reaction basin, release of phosphorus is taken from activated sludge. Microorganisms take up the released phosphorus components luxuriously in the aerobic reaction basin. The microorganisms having done the luxury uptake are removed by the waste sludge, thereby finally nitrogen and phosphorus are removed.
In a conventional method for removing nitrogen and phosphorus, anaerobic basins, anoxic basins and aerobic basins are separately installed with a constant capacity. Thus, it is not possible to cope with changes in quality and amount of influent with flexibility. Also, methanol must be injected into the plant as an electron donor form denitrification, or water in a nitrification basin must be recirculated to the previous phase, that is a denitrification basin, to utilize organisms contained in the waste water. For injection of methanol it costs a lot for chemicals, and for utilizing the organisms, the costs for installation of pump, electricity, maintenance and management are in need, since the recirculation flow is about 4 times of the influent.
To overcome such problems, methods of intermittent aeration and flow path change have been proposed. As typical conventional arts employing the intermittent aeration and flow path change, there is a so called Bio Denipho process or a Phased Isolation Ditch (PID) process.
FIGS. 7A and 7B show a conventional PID process form removing nitrogen and phosphorus, illustrating changes in inflow and outflow directions, that is, flow path changes, in a aerated or anerated state for the respective phases A through D.
The overall configuration of an equipment for the PID process will be described in the order of progress hereinbelow. In view of the progress of the influent, the equipment is constructed by a preliminary denitrification basin 201a, a selection basin 201b, an anerobic basin 201c, at least two sets of oxidation ditches 202 and 203 each having an aerator and a mixer, and a clarifier 204 having a sludge collector 206. Also, there are equipped with a sludge return pump 205 for returning the sludge from the clarifier 204 to the preliminary denitrification basin 201a, and a sludge return pipe 208.
The function of the anaerobic basin 201c is to mix incoming raw waste water with the sludge returned from the clarifier 204 by the sludge return pump 205 and to release the phosphorus from the sludge while an anaerobic state is maintained. If chemically combined oxygen such as nitrate (NO.sub.3) or nitrite (NO.sub.2) exists, the release of phosphorus from the sludge hardly occurs. Thus, in the previous phase of the anaerobic basin 201c, free oxygen or nitrate contained in the raw waste water or sludge returned is first removed from the preliminary denitrification basin 201a and the selection basin 201b. Otherwise, the anaerobic basin 201c shall be constructed with at least two sets of basins in combination to prevent short circuit, and a mixer 301 is installed in each reaction basin.
The clarifier 204 is an external clarifier independently installed outside the oxidation ditches 202 and 203, and the sludge collector 206, the sludge return pump 205 and return pipe 208 are additionally installed therein. Here, the sludge return flow must be more than the total amount of the inflow.
As described above, in view of facility, maintenance and management, the PID process requires installation costs, electricity and facility management costs due to the construction of the preliminary denitrification basin 201a, the selection basin 201b, the anaerobic basin 201c and the clarifier 204, and the installation and operation of the sludge return pump 205, the sludge collector 206 and the mixer 301.
Also, in view of a processing efficiency, in the PID process, since a change between phases is not fast and is not clear, the processing efficiency may be lowered. As the activated sludge having a reduced content of phosphorus by the release of phosphorus in an anaerobic state, is converted to be in an aerobic state so that the microorganisms are activated, These activated microorganisms take up phosphorus luxuriously. However, in the PID process, the sludge passed through the release of phosphorus in the anaerobic basin 201c is induced to an anoxic state in phases A and C, not to an aerobic state. Thus, the microorganisms may not be sufficiently activated, and this lowers the uptake efficiency of phosphorus.
During a denitrification process, to reduce nitrogen oxide, organisms are required as electron donors as much as possible. However, in the PID process, the sludge on which a large amount of organisms required for denitrification are adsorbed is continuously release from the oxidation ditch in which denitrification is carried out in the anoxic state, thus the sludge is induced to the oxidation ditch where a nitrification process in which a large amount of the incoming organisms acts unfavorably, is carried out. Hence, much more time is required for the nitrification process and the denitrification efficiency of the anoxic oxidation ditch is lowered.
In the phase A in PID process, the same amount of sludge as that of inflow is continuously released from the first oxidation ditch 202 where denitrification is carried out, then flows into the second oxidation ditch 203 where nitrification is carried out. Thus, the organisms adsorbed in the sludge are washed away from the first oxidation ditch 202, which is unfavorable to the denitrification. Then, the organisms are introduced to the second oxidation ditch 203 where nitrification is carried out to act unfavorably on the nitrification.
These situations also occur in the phase C in which the flow path is changed and denitrification is carried out in the second oxidation ditch 203. the oxidation ditch constituting the unit system including the clarifier from which the sludge returned is drawn out. In other words, the sludge returned is not flown into the oxidation ditch of the adjacent unit system. Also, according to the present invention, the flow paths of the waste water can be changed and means for controlling the flow paths is provided.